The present invention relates to intracellular receptors, nucleic acids encoding same, and uses therefor. In a particular aspect, the present invention relates to methods for the modulation of physiological response to elevated levels of steroid and/or xenobiotic compounds.
Nuclear receptors constitute a large superfamily of ligand-dependent and sequence-specific transcription factors. Members of this family influence transcription either directly, through specific binding to the promoters of target genes (see Evans, in Science 240:889-895 (1988)), or indirectly, via proteinxe2x80x94protein interactions with other transcription factors (see, for example, Jonat et al., in Cell 62:1189-1204 (1990), Schuele et al., in Cell 62:1217-1226 (1990), and Yang-Yen et al., in Cell 62:1205-1215 (1990)). The nuclear receptor superfamily (also known in the art as the xe2x80x9csteroid/thyroid hormone receptor superfamilyxe2x80x9d) includes receptors for a variety of hydrophobic ligands, including cortisol, aldosterone, estrogen, progesterone, testosterone, vitamin D3, thyroid hormone and retinoic acid, as well as a number of receptor-like molecules, termed xe2x80x9corphan receptorsxe2x80x9d for which the ligands remain unknown (see Evans, 1988, supra). These receptors all share a common structure indicative of divergence from an ancestral archetype.
Lipophilic hormones such as steroids, retinoic acid, thyroid hormone, and vitamin D3 control broad aspects of animal growth, development, and adult organ physiology. The effects of these hormones are mediated by a large superfamily of intracellular receptors that function as ligand-dependent and sequence-specific transcription factors. The non-steroidal nuclear receptors for thyroid hormone (TR), vitamin D3 (VDR), all-trans retinoic acid (RAR), and fatty acids and eicosanoids (PPAR) form heterodimers with the 9-cis retinoic acid receptor (RXR) that bind bipartite hormone-response elements (HREs) composed of directly repeated half sites related to the sequence AGGTCA (Mangelsdorf and Evans, Cell 83: 841-850, 1995). In contrast, the steroid receptors function as homodimers and bind to palindromic target sequences spaced by three nucleotides (Beato et al., Cell 83: 851-857, 1995). In addition to the known receptors, a large group of structurally-related xe2x80x9corphanxe2x80x9d nuclear receptors has been described which possess obvious DNA and ligand binding domains, but lack identified ligands (Mangelsdorf et al., Cell 83:835-839, 1995; Enmark and Gustafsson, Mol. Endocrinol. 10:1293 (1996); and O""Malley and Conneely, Mol. Endocrinol. 6:1359 (1992)). Each has the potential to regulate a distinct endocrine signaling pathway.
It is widely viewed that the hormone response is a consequence of the release, from an endocrine gland, of a ligand that circulates through the blood, and coordinately regulates responses in target tissues by acting through specific nuclear receptors. Hormone responsiveness is dependent on the ability to rapidly clear ligand from the blood and the body so that, in absence of a stimulus, target tissues return to a ground state. Hormonal homeostasis is thus achieved by the coordinated release and degradation of bioactive hormones. Steroid hormones and their many metabolites are primarily inactivated by reduction and oxidation in the liver. Since hundreds of adrenal steroids have been identified (e.g., dozens of each of the sex steroids (androgens, estrogens and progestins), 25-35 vitamin D metabolites, and likely hundreds of fatty acids, eicosanoids, hydroxyfats and related bioactive lipids), the problem of efficient ligand elimination is critical to physiologic homeostasis. In addition to the existence of a myriad of endogenous hormones, a similar diversity of ingested plant and animal steroids and bioactive xenobiotic compounds must also be degraded.
Selye first introduced the concept that exogenous steroids and pharmacologic substances may function to modulate the expression of enzymes that would protect against subsequent exposure to toxic xenobiotic substances (H. Selye, J. Pharm. Sci. 60:1-28, 1971). These compounds, which Selye called xe2x80x9ccatatoxic steroids,xe2x80x9d are typified by the synthetic glucocorticoid antagonist, pregnenolone-16-carbonitrile (PCN). PCN, and a variety of xenobiotic steroids, induce the proliferation of hepatic endoplasmic reticulum and the expression of cytochrome P450 genes (Burger et al., Proc. Natl. Acad. Sci. (USA) 89:2145-2149, 1992; Gonzalez et al., Mol. Cell. Biol. 6:2969-2976, 1986; and Schuetz and Guzelian, J. Biol. Chem. 259:2007-2012, 1984). One consequence of PCN treatment is the induction of nonspecific xe2x80x9cprotectionxe2x80x9d against subsequent exposure to such diverse xenobiotics as digitoxin, indomethacin, barbiturates, and steroids (Selye, supra, 1971).
Furthermore, it is known that a variety of such compounds can activate P450 genes responsible for their detoxification or degradation (Fernandez-Salguero and Gonzalez, Pharmacogenetics 5:S123-128, 1995; Denison and Whitlock, J. Biol. Chem. 270:18175-18178, 1995; O. Hankinson, Ann. Rev. Pharmacol. Toxicol. 35:307-340, 1995; and Rendic and Di Carlo, Drug Metab. Rev. 29:413-580, 1997).
While it appears that such catatoxic compounds regulate the expression of cytochrome P450s and other detoxifying enzymes, two lines of evidence argue that such regulation is independent of the classical steroid receptors. First, many of the most potent compounds (e.g., PCN, spironolactone, and cyproterone acetate) have been shown to be steroid receptor antagonists; whereas others (e.g., dexamethasone) are steroid receptor agonists (Burger, supra, 1992). Second, the nonspecific protective response remains after bilateral adrenalectomy (and presumably in the absence of adrenal steroids), but not after partial hepatectomy (Selye, supra, 1971).
Insight into the mechanism by which PCN exerts its catatoxic effects is provided by the demonstration that PCN induces the expression of CYP3A1 and CYP3A2, two closely related members of the P450 family of monooxygenases (see, for example, Elshourbagy and Guzelian in J. Biol. Chem. 255:1279 (1980); Heuman et al., in Mol. Pharmacol. 21:753 (1982); Hardwick et al., in J. Biol. Chem. 258:10182 (1983); Scheutz and Guzelian in J. Biol. Chem. 259:2007 (1984); Scheutz et al., in J. Biol. Chem. 259:1999 (1984); and Gonzalez et al., in J. Biol. Chem. 260:7435 (1985)). The CYP3A hemoproteins display broad substrate specificity, hydroxylating a variety of xenobiotics (e.g., cyclosporin, warfarin and erythromycin), as well as endogenous steroids (e.g., cortisol, progesterone, testosterone and DHEA-sulfate. See, for example, Nebert and Gonzalez in Ann. Rev. Biochem. 56:945 (1987) and Juchau in Life Sci. 47:2385 (1990)). A PCN response element (which is highly conserved in the CYP3A2 gene promoter) has since been identified in subsequent studies with the cloned CYP3A1 gene promoter (see Miyata et al., in Archives Biochem. Biophysics 318:71 (1995) and Quattrochi et al., in J. Biol. Chem. 270:28917 (1995)). This response element comprises a direct repeat of two copies of the nuclear receptor half-site consensus sequence AGTTCA.
In addition to inducing CYP3A gene expression, PCN has also been shown to have marked effects on hepatic cholesterol homeostasis. These effects include significant decreases in the levels of HMG-CoA reductase and cholesterol 7a-hydroxylase gene expression, with associated reductions in sterol biosynthesis and bile acid secretion. PCN has also been reported to enhance the formation of cholesterol esters and the hypersecretion of cholesterol into the bile. Thus, PCN affects key aspects of cholesterol metabolism, including its biosynthesis, storage and secretion.
Activation of orphan nuclear receptor(s) by catatoxic steroids provides a possible mechanism for the induction of xenobiotic metabolizing enzymes by compounds that do not activate known steroid receptors. Because such enzymes are activated by high (pharmacological) doses of xenobiotic and natural steroids, such a xe2x80x9csensorxe2x80x9d would be expected to be a broad-specificity, low-affinity receptor. Such receptors could be activated not only by endogenous steroids and metabolites but also by exogenous compounds such as phytosteroids, xenobiotics and pharmacologic inducers. Indeed, it is known that a variety of such compounds can activate P450 genes responsible for their detoxification or degradation (see, for example, Fernandez-Salguero and Gonzalez in Pharmacogenetics 5:S123 (1995); Denison and Whitlock, Jr. in J. Biol. Chem. 270:18175 (1995); Hankinson in Ann. Rev. Pharmacol. Toxicol. 35:307 (1995); and Rendic and Di Carlo in Drug Metab. Rev. 29:413 (1997)).
In healthy individuals, steroid levels are tightly regulated, with increased catabolism of endogenous steroids being compensated by the pituitary releasing an increase of ACTH, which stimulates biosynthesis, and maintenance of plasma steroid levels. The increased catabolism is reflected by elevated urinary levels of steroid metabolites. Indeed, it is already known that treatment with rifampicin increases urinary metabolites, such as 6xcex2-hydroxycortisol (Ohnhaus et al., Eur. J. Clin. Pharmacol. 36:39-46, 1989; and Watkins et al., J. Clin. Invest., 83:688-697, 1989), and bile acid metabolites, such as 6xcex2-hydroxy hyocholic and 6xcex1-hyodeoxycholic acids (Wietholtz et al., J. Hepatol, 24:713-718, 1996), while the plasma levels of many circulating steroids rise slightly due to increased synthesis (Lonning et al., J. Steroid Biochem. 33:631-635, 1989; Bammel et al., Eur. J. Clin. Pharmacol, 42:641-644, 1992; and Edwards et al., Lancet 2:548-551, 1974).
When synthetic steroids, such as prednisolone (McAllister et al., Br. Med. J. 286:923-925, 1983; and Lee et al., Eur. J. Clin. Pharmaco. 45:287-289, 1993) or 17xcex1-ethynylestradiol (F. P. Guengerich, Life Sci., 47:1981-1988, 1990) are administered together with rifampicin, plasma levels are rapidly decreased due to enhanced urinary clearance. In some patients undergoing rifampicin therapy for tuberculosis, the increase in urinary steroid levels has led to misdiagnosis of Cushing""s syndrome (Kyriazopoulou and Vagenakis, J. Clin. Endocrinol. Metab., 75:315-317, 1992; Zawawi et al., Ir. J. Med. Sci., 165:300-302, 1996; and Terzolo et al., Horm. Metab. Res., 27:148-150, 1995). In these patients, steroid production and clearance normalized when rifampicin was withdrawn. In patients with Addison""s disease, who mostly lack the ability to synthesize adrenal steroids, rifampicin treatment leads to rapid depletion of endogenous and administered steroids. These documented clinical situations confirm that induction of CYP3A4 causes increased steroid catabolism (Kyriazopoulou et al., J. Clin. Endocrinol Metab. 59:1204-1206, 1984; and Edwards, supra, 1974). However, the art is silent regarding the mechanism by which steroid metabolism is regulated in the body.
Although therapeutically administered steroids are beneficial in achieving therapeutic goals, such compounds can, in some cases, increase the overall level of steroids and xenobiotics above physiologically compatible levels in the subjects to whom they are administered. In other cases, the increased level of steroids and/or xenobiotics may linger in the body longer than is therapeutically required. In addition, some subjects are treated with combinations of steroids and xenobiotics that may be administered separately to treat different conditions, but which, in combination, have an additive, or even synergistic, effect known as a drug interaction. In such cases, the patient may be unaware when a physiologically incompatible level of steroids and xenobiotics has been reached, or when an otherwise therapeutic amount of a steroid becomes potentially dangerous due to combined effects of separately administered drugs.
Accordingly, there is still a need in the art for the identification and characterization of broad specificity, low affinity receptors that participate in the mediation of the physiological effect(s) of steroids and xenobiotics, particularly when combinations of such compounds disrupt homeostasis or cause drug interaction.
In accordance with the present invention, we have isolated and characterized an example of a novel class of human orphan nuclear receptor, termed the steroid and xenobiotic receptor (SXR). SXR is expressed almost exclusively in the liver, the primary site of xenobiotic and steroid catabolism. Unlike classical steroid receptors, SXR heterodimerizes with RXR and binds to directly repeated sequences related to the half-site, AGTTCA. SXR can activate transcription through response elements found in some steroid inducible P450 genes in response to an enormous variety of natural and synthetic steroid hormones, including antagonists such as PCN, as well as xenobiotic drugs, and bioactive dietary compounds, such as phytoestrogens. The ability of SXR to regulate expression of catabolic enzymes in response to this diversity of steroid and/or xenobiotic compounds provides a novel mechanism for direct regulation of metabolism so as to achieve physiologic homeostasis with respect to such steroid and/or xenobiotic compoundsxe2x80x94ideal properties for a xe2x80x9csteroid sensing receptorxe2x80x9d which mediates the physiological effect(s) of hormones. SXR represents the first new class of steroid receptors described since the identification of the mineralocorticoid receptor ten years ago.
In accordance with a particular aspect of the present invention, there are also provided nucleic acid sequences encoding the above-identified receptors, as well as constructs and cells containing same, and probes derived therefrom. Furthermore, it has also been discovered that a wide variety of substrates modulate the transcription activating effects of invention receptors.
An important requirement for physiologic homeostasis is the removal and detoxification of various endogenous hormones and xenobiotic compounds with biological activity. Much of the detoxification is performed by cytochrome P450 enzymes, many of which have broad substrate specificity and are inducible by a bewildering array of compounds, including steroids. The ingestion of dietary steroids and lipids induces the same enzymes and, thus, must be integrated into a coordinated metabolic pathway. Instead of possessing hundreds of receptors, one for each inducing compound, a class of broad-specificity, low-affinity nuclear receptors has been discovered that monitor total steroid levels and induce the expression of genes encoding xenobiotic metabolizing enzymes. SXR, which is a member of a novel branch of the nuclear receptor superfamily, forms part of a steroid sensor mechanism for removal of elevated levels of steroids and/or xenobiotic compounds from circulation via broad-specificity, low-affinity receptors that represent a novel branch of the nuclear receptor superfamily.